It is necessary to monitor the quality of water used in facilities using an online monitoring apparatus in order to stably and effectively operate a water utilizing facility. In such a monitoring apparatus, for example, the concentration of a specific dissolved substance in the sample water is automatically measured by absorptiometry. In that case, in the absorptiometry, it is necessary to make measuring waters in which colors are developed by respectively adding, for example, a plurality of kinds of reagents to sample waters, and transmit lights to the measuring waters.
However, in such a monitoring apparatus, it may cause a case wherein a part of reagents is not added to the sample water because of a bad condition of a reagent pump or the depleting of the reagent. This may cause not only a wrong judgement of the quality of the sample water due to a lack of a correct measuring result, but also a problem of damaging the apparatus under the effect of other reagents.
Meanwhile, as for a method for preventing non-addition of the reagent, for example, there can be considered a method for providing a device to detect a remaining quantity of the reagent to a reagent bottle (Patent Document 1); a method for estimating the remaining quantity of the reagent from an operating time of an apparatus sending out the reagent (for example, the reagent pump); or a method for judging the presence or absence of addition of the reagent on the basis of the presence or absence of an indicator by adding the indicator (for example, a coloring agent), which develops a color different from a color developed by the reagent, to the reagent (Patent Documents 2 and 3).